Curves of growth for transiting exocomets: Application to Fe II lines in the Beta Pictoris system

Abstract

This study introduces the exocomet curve of growth, a new method to analyse the variable absorptions observed in $\beta$ Pictoris spectrum and link them to the physical properties of the transiting cometary tails. We show that the absorption depth of a comet in a set of lines arising from similar excitation levels of a given chemical species follows a simple curve as a function of the gf-values of the lines. This curve is the analogue of the curve of growth for interstellar absorption lines, where equivalent widths are replaced by absorption depths. To fit this exocomet curve of growth, we introduce a model where the cometary absorption is produced by a homogeneous cloud, covering a limited fraction of the stellar disc. This model is defined by two parameters: $\alpha$, the covering factor of the cloud, and $\beta$, related to its typical the optical depth. This model is tested on two comets observed with the Hubble Space Telescope in December 1997 and October 2018, in a set of Fe II lines at 275 nm. The measured absorption depths are found to satisfactory match the two-parameter curve of growth model, indicating that both comets cover roughly 40 % of the stellar disc ($\alpha =0.4$) and have optical thicknesses close to unity. Then, we show that if we consider a set of lines arising from a wider range of energy levels, the absorbing species seems to be populated at thermodynamical equilibrium, causing the cometary absorption to follow a curve of growth as a function of $gf\cdot e^{-E_l/k_{B}T}$ (where T is the temperature of the absorbing medium). For the comet observed on December 6, 1997, we derive a temperature of $10500\pm 500$ K and a total Fe II column density of $(1.11\pm 0.09)\times 10^{15}$ cm${}^{-2}$. By probing the population of the highest excited energy levels ($E_l\sim 25000$ cm${}^{-1}$), we also estimate an electronic density of $(3\pm 1)\times 10^7$ cm${}^{-3}$.

19 pages, 16 figures

References in corpus (11)

• The Carbon-Rich Gas in the Beta Pictoris Circumstellar Disk
• Direct confirmation of the radial-velocity planet $\beta$ Pic c
• Dynamical traceback age of the $\beta$ Pictoris moving group
• The mass of Beta Pictoris c from Beta Pictoris b orbital motion
• Transiting exocomets detected in broadband light by TESS in the $\beta$ Pictoris system
• The Inner Disk Structure, Disk-Planet Interactions, and Temporal Evolution in the Beta Pictoris System: A Two-Epoch HST/STIS Coronagraphic Study
• Exocomets size distribution in the $\beta$ Pictoris planetary system
• Volatile-Rich Circumstellar Gas in the Unusual 49 Ceti Debris Disk
• Exocomets from a Solar System Perspective
• New exocomets of $\beta$ Pic
• Hint of an exocomet transit in the CHEOPS lightcurve of HD 172555

Cited by in corpus (5)

• Population of excited levels of Fe+, Ni+ and Cr+ in exocomets gaseous tails
• Abundances of refractory ions in Beta Pictoris exocomets
• Exocomets of $\beta$ Pictoris I: Exocomet destruction, sodium and disk line variability in 17 years of HARPS observations
• Transit distances and composition of low-velocity exocomets in the $\beta$ Pic system
• Potential sublimating exocomets around the young star PDS 70